This invention relates generally to high power solid state lasers and, more particularly, to solid state laser amplifiers of the zig-zag type, in which a light beam is confined laterally in a solid state slab of doped material, and is subject to repeated internal reflections from parallel faces of the slab. A zig-zag laser amplifier of the prior art is exemplified by the one disclosed in U.S. Pat. No. 6,094,297, to Injeyan et al., entitled “End Pumped Zig-Zag Slab Laser Gain Medium.”
In the configuration described in U.S. Pat. No. 6,094,297, a slab of solid state lasing material employs end pumping, which is to say that a pump beam is launched into one or both ends for the slab, and a relatively high overall efficiency is attained by passing the amplified light through the slab in a zig-zag manner. The pump energy is launched into the slab as a narrow beam that passes through one of the parallel faces of the slab and reflects from an end face angled at 45° to the parallel faces. The light to be amplified is launched into the slab through one of the end faces and is internally reflected back from the parallel faces as it progresses along the slab and absorbs energy from the pump beam.
As shown in more detail in FIG. 1, a zig-zag optical amplifier of the prior art is generally identified with the reference numeral 20. The optical amplifier 20 utilizes end pumping, which means that pumped light is generally co-aligned with the amplified light along a longitudinal axis of the slab, resulting in a relatively long absorption length, and thus providing relatively high overall efficiencies. Typically, the configuration is particularly suitable for optical amplifiers that utilize solid state lasing material with relatively low absorption coefficients, such as those materials using neodymium (Nd), ytterbium (Yb), and thulium (Tm) as dopants. The absorption of the pumped light may be confined to a central region of the slab to reduce heating and possible optical distortions at opposing ends of the slab.
More specifically, the optical amplifier 20 of the prior art includes an elongated slab 22 and a pair of pumped beam sources 21 and 26. The elongated slab 22 is formed with a generally rectangular or square cross section defining a pair of opposing end faces 28 and 30 and four lateral faces 32. A longitudinal or lasing axis 34 is defined as an axis generally parallel to the lateral faces 32 and extending between the opposing end faces 28 and 30. A major axis is defined as a horizontal axis in the direction of the zig-zag pattern while a minor axis is defined to be a vertical axis generally perpendicular to the major axis. Both the major and minor axis are perpendicular to the longitudinal axis. The FIG. 1 view is generally considered as a top plan view, in the direction of the minor axis.
The slab 22 may be formed from a solid state lasing material with a relatively high index of refraction to cause internal reflection of the input beam in a generally zig-zag pattern as illustrated in FIG. 1, forming a so called zig-zag amplifier. Such zig-zag amplifiers are known to allow brightness scaling by allowing the input beam to average thermal gradients in the slab, effectively providing a homogeneous gain medium. In order to reduce heating of the ends of the slab 22, the slab may be formed as a diffusion bonded composite material. More particularly, along the longitudinal axis 34 of the slab 22, the opposing end portions 34 and 36 of the slab 22 can be formed from undoped host materials, such as yttrium-aluminum-garnet (YAG). These end portions 34 and 36 can be diffusion bonded to a central portion 38 of the slab 22 formed from a doped host material, such as Nd or Yb doped YAG (Nd:YAG or Yb:YAG), forming two diffusion bond interfaces 40 and 42. Such a configuration limits the absorption length to the center portion 38 of the slab 22. By limiting the absorption length to the center portion 38 of the slab 22, heat generated by the optical pumping is generally confined to the center portion and away from the end portions 34 and 36, which may not have cooling and are thus susceptible to thermal distortion. As mentioned above, the pumped light is reflected through the slab 22. The pump beams 21 and 26 may enter opposing lateral faces 32 of the slab 22 at opposing end portions 34 and 36, respectively, as generally shown in FIG. 1. In order to enable the light into the slab 22, one or more footprints or windows 41 and 43 may be formed on opposing end portions 34 and 36. The windows 41 and 43 may be formed by way of a coating, such as an antireflection coating selected for the wavelength of the pump beams 21 and 26. As also shown in FIG. 1, the antireflection coating is disposed on the lateral face 32 as well as the opposing end faces 28 and 30, thereby reducing losses of the input beam and pump beam. The pump beams 21 and 26 are directed to opposing lateral faces 32 at opposing end portions 34 and 36 of the slab 32. The pump beams 21 and 26 are totally reflected from the opposing end faces 28 and 30 so that the pump beams are co-aligned with the longitudinal axis 34. By utilizing the composite slab 22 as discussed above, the absorption length of the slab 22 is limited to the central portion 28.
An input light beam 44 is directed into one end face 28 at a relatively small angle, for example, less than 20° relative to the normal of the end face. By proper selection of the angle of incidence of the input beam 44 and selecting a material having a relatively high index of refraction, the input beam is totally reflected along the slab 22 in the generally zig-zag pattern as shown and is out coupled as an amplified beam 46, through the opposing end face 30. The zig-zag pattern across the slab temperature gradients, combined with uniform pumping by the guided diode light and insulated slab edge, results in relatively low thermal lensing with virtually no birefringence.
The laser amplifier of FIG. 1 is relatively efficient in terms of its utilization of pump beams co-aligned with the optical axis of the slab 22, and provides an output with good beam quality and polarization properties. However, the total output power of this device is limited by its small cross sectional area, which is in turn limited by the need to keep the slab thickness small (a few millimeters) to provide adequate removal of residual heat deposited in the slab. Coherent scaling of multiple amplifiers of this type would require many such slabs to reach very high powers. Consequently, the size, weight, complexity, and cost of such a system would be excessive for many applications.
In the end-pumped zig-zag amplifier of FIG. 1, the beam to be amplified and injected from one end experiences total internal reflection (TIR) alternately at the faces of the slab. In the prior art the slab faces are coated with an ‘evanescent wave coating,’ as described in U.S. Pat. No. 4,881,233. This is a uniform film deposited on the slab surface that has an index lower than the slab material. The amplified beam's field decays exponentially in the evanescent coating such that there is negligible field present at the coating surface. Conduction, liquid, or other cooling can then be applied to the coated slab surfaces to remove excess heat from the slab without impacting the optical performance. Temperature gradients that form within the slab can induce refractive index nonuniformities and birefringence. However, the alternate traversals (zig-zags) of the beam tend to average out these effects and maintain both good beam quality; i.e., uniform optical path difference (referred to as OPD) and polarization purity. The temperature variation in the slab, and therefore the amount of OPD and birefringence, can be minimized by keeping the slab thickness small. In the device of U.S. Pat. No. 6,094,297, as illustrated in FIG. 1, rather than side pumping through a thin slab, end pumping was utilized to achieve an adequate path length and absorption of the pump light, and the lateral faces could then be cooled with an opaque solid conductive cooler. When such a slab amplifier is operated as part of a MOPA (Master Oscillator Power Amplifier) architecture, many amplified beams can be coherently phased and combined by use of prior art techniques, for example as described in U.S. Pat. Nos. 6,404,784 and 6,219,360. This approach, however, suffers from large volume and weight requirements, when scaled to incorporate many slabs.
To minimize size and weight, one ideally would prefer to generate 100 kW or greater output within a single small optical aperture of approximately 1.0 sq. inch (6.5 cm2). Another prior art approach, referred to as the ‘liquid laser,’ is described in International Patent Publication No. WO 03/047052 and uses an amplifier comprised of many thin gain plates with intervening flowing liquid coolant channels, where the amplified beam passes through both the plates and the liquid. Although this scheme is effective at mitigating the OPD within the plates, a major difficulty is encountered from the OPD produced by temperature gradients in the flowing liquid.
In summary, any attempt to scale the device of FIG. 1 to higher output powers is rendered extremely difficult by the sheer bulk associated with multiple devices of this type, each of which must be cooled and have its output beam combined with others to produce a high power composite output. Other approaches to attaining high output powers from solid state lasers also have serious shortcomings.
The zig-zag laser described was a significant advance over another prior art technique that employed side pumping of a zig-zag slab laser. In side pumping, pump laser diodes are arrayed across one or both parallel side faces of the slab and direct the pump energy perpendicular to the principal direction of the light beam being amplified in the slab. One serious problem with this approach was that the effective path length of the pump beams was limited by the thickness of the slab. For a very thin slab, only a small proportion of the pump energy could be absorbed. Increasing the slab thickness allowed for more pump energy to be absorbed, but the thicker slab had significant thermal gradient problems. The side-pumped approach of the prior art is exemplified by the disclosure of U.S. Pat. No. 4,881,233, to von Arb et al., entitled “Laser with Improved Cooling System.” The end-pumping approach of U.S. Pat. No. 6,094,297 provided a longer path for the pump beams, and consequently a more efficient device.
Although the end-pumped zig-zag configuration makes relatively efficient use of pump power, it does not satisfy the need for a solid state laser architecture that is scalable to much higher powers, on the order of 100 kw or higher. Combining multiple zig-zag lasers of the type shown in U.S. Pat. No. 6,094,297 results in an extremely bulky structure. Each such laser has to be cooled along its parallel faces, and pump light must be focused into the small end faces of each slab, which enders it next to impossible to place them in close proximity to each other. Combining the output beams of the multiple lasers is also rendered difficult by the bulk and necessary spacing of the individual devices. Moreover, the resulting structure would be impractically large for the powers desired.. Ideally, a high-power laser for military or commercial applications should be conveniently portable, or at least movable on a conventional vehicle.
Therefore, it will be appreciated that there is still a need for a high-power solid state laser amplifier structure that meets these requirements. That is to say, there is a need for a sold state laser that is scalable to very high powers, but which is extremely and conveniently compact, to facilitate transport of the device, and to facilitate combination of its multiple output beams into a single high-power composite beam. The present invention achieves these goals, as will shortly become apparent from the following summary and the more detailed description.